A nebulizer is a drug delivery system by aerosol into the lungs, and is used to treat diseases such as Cystic Fibroses, COPD and Asthma. A number of companies make devices for respiratory drug delivery by aerosol. Preferably the devices are compact, portable, battery operated and lightweight.
Nebulizers generate an aerosol flow, and the patient receives a specific amount of medication in the form of small droplets (aerosol) that are typically formed by forcing the medication through a mesh in the form of a thin metal plate with tiny holes.
The volume of medication to be nebulized (typically 0.2 to 2 ml) is dosed into the device, and the device generates the aerosol by means of well known methods such as a vibrating mesh as mentioned above, or a vibrating horn, or vibrating flat plate in a resonant cavity. The required ultrasonic vibration is generated by an actuator, typically a piezoelectric crystal. The amount of medication that reaches the patient during the treatment is equal to the supplied medication dose minus the aerosol deposited in the device and residues of medication that remain in the device after the treatment is finished.
The proper medical dosage of a nebulizer is essentially dependent on the output volume, but correct application of the dose can also depend on the particle size of the aerosol in which the drug is dissolved. The output volume varies with aging of the output mesh of the nebulizer because the mesh will deteriorate over time, for example due to clogging of the thousands of tiny holes (˜2.5 micron diameter conical holes). The viscosity of the medicine may also change with temperature, and hence change the output.
The breathing pattern of the patient is also of importance. In current systems, aerosol density and particle size are not measured, let alone used to give feedback to the system or patient. This may lead to under dosage, over dosage, waste of drug, unnecessary contamination of the environment and higher costs.
For a medication therapy, it is sometimes required that not only the dose is precisely defined, but also the rate at which the medication is delivered, namely the aerosol output rate. The nebulizer generally controls the aerosol output rate by means of the electrical power and driving frequency applied to the piezoelectric drive system.
The aerosol output rate cannot be exactly predicted based on the applied electrical power. Aerosol generating systems may have different efficiencies (amount of aerosol generated per unit electrical power), for example due to device and mesh tolerances, temperature, and cleanliness of the mesh.
A system has been proposed that estimates the aerosol output rate by measuring the density of the aerosol beam, which is then used in a feedback control loop to adjust the electrical power. The aerosol density can be measured by means of an optical beam perpendicular to the aerosol beam. The optical beam can be generated by a light emitting diode (LED). The beam shape of the light from a LED is divergent, and the optical beam may be collimated to a parallel or nearly parallel beam using one or more lenses or mirrors. The beam may be further shaped using a circular or rectangular diaphragm.
The optical beam crosses the aerosol beam, and falls on an optical sensor (optionally through a diaphragm and optionally focused using one or more lenses). The optical system can be calibrated by measuring the sensor signal at a time that no aerosol is present with the LED off (“dark signal”) and with the LED on (“light signal”). If the aerosol beam is present, the rays of the optical beam are scattered by the droplets, thus decreasing the light that falls on the optical sensor, and hence decreasing the measured output signal at the optical sensor. The decrease of light on the sensor caused by droplets in the light path is called obscuration. The obscuration can be quantitatively expressed by the parameter (“light signal”−“measured signal”)/(“light signal”−“dark signal”).
The obscuration is a function of the droplet density in the aerosol beam and the length over which the light travels through the aerosol beam. If the velocity of the aerosol beam is known, e.g. through a separate air flow rate measurement (using a differential pressure sensor or a flow sensor), then the aerosol output rate can be computed from the aerosol density and the volume of the aerosol beam that passes the optical beam per unit of time. The volume can be calculated from the product of the cross-sectional area of the aerosol beam and the velocity of the aerosol beam.
The level of obscuration by itself does not give any indication of aerosol density nor particle size. Only if the particle size is known can the aerosol density be derived from the obscuration. In practice, the nominal particle size is often mostly predetermined by the design and construction of the complete aerosol generator.
However, it is desirable to know the particle size, either to give an indication of the performance of the aerosol generating system (for example to provide an indication of ageing) or because certain particle sizes are desired for particular absorption characteristics, so that particle size becomes a parameter which characterizes the performance of the system.